Electronic interface for LVDT-type pressure transducers using piezoresistive sensors

ABSTRACT

Aspects of the present disclosure relate to a method that includes receiving, at first and second input terminals, first and second input voltages and maintaining the direction of the first and second input voltages and doubling the first and second input voltages. The method also includes measuring, by a Wheatstone bridge, an applied physical parameter; outputting, to a capacitor, and to first and second operational amplifiers, first and second signals substantially indicative of the physical parameter; reducing, by the capacitor, a DC component of the first and second signals; amplifying, by the first and second operational amplifiers, the first and second signals; and adjusting, by a gain resistor, a differential output between first and second output voltages that correspond to the first and second signals output from the Wheatstone bridge. Finally, the method includes outputting, by first and second output terminals, the first and second output voltages.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of and claims priority under 35U.S.C. 120 to U.S. patent application Ser. No. 13/356,930, which wasfiled on Jan. 24, 2012. U.S. patent application Ser. No. 13/356,930claims the benefit of U.S. Provisional Patent Application No.61/438,564, which was filed on Feb. 1, 2011. The entire contents andsubstance of each of these applications are hereby incorporated byreference in their entirety as if fully set forth herein.

TECHNICAL FIELD

The present disclosure relates to LVDT-type pressure transducerassemblies.

BACKGROUND

A Linear Variable Differential Transformer (LVDT) is a type ofelectrical transformer used for measuring linear (i.e. translational)displacement. Linear variable differential transformers, as illustratedin FIG. 1 (prior art), are often used as physical sensing elements inelectronic circuits to provide an electrical measurement of smallphysical displacements, such as those produced by linear movements orpressure changes.

Early LVDT pressure transducers included mechanical components, such asan aneroid or a Bourdon tube, as illustrated in FIG. 2. The pressureapplied to the aneroid caused a displacement of the core of the LVDT,thereby generating an output ratio R proportional with the appliedpressure. The pressure was then determined by measuring the two outputvoltages V1 and V2 of the LVDT and calculating the ratio R. These LVDTpressure transducers had significant shortcomings, however, primarilydue to the mechanical nature of their components. These shortcomingstypically included poor accuracy, poor stability, sensitivity tovibration and shocks, and large hysteresis, i.e., sticking, due to themechanical friction of the core. As a result, they were subject totransducer failure and erroneous outputs, particularly when subjected tohigh-vibration environments and acceleration forces. Therefore, whileconventional mechanical LVDTs are still commonly used, it is desirableto replace them with improved LVDT-type sensing circuits that are lessprone to transducer failure and erroneous output.

Solid-state implementations of LVDT-type pressure transducers have beenintroduced to obviate many of the shortcomings of their mechanicalcounterparts. One solid-state LVDT-type transducer uses a piezoresistivebridge for pressure sensing and an electronic circuit to generateLVDT-type output, as described in U.S. Pat. No. 5,398,194 (“ELECTRONICSENSING CIRCUIT USING PIEZORESISTORS,” issued to Amnon Brosh et al. onMar. 14, 1995 and assigned to Kulite Semiconductor Products, Inc.,Leonia, N.J.). FIG. 3 provides a schematic of an implementation of onesuch solid-state circuit. While such solid-state traducers eliminatemany of the shortcomings of mechanical LVDT-type pressure sensors, theyhave drawbacks of their own. For example, it has been observed thatthese solid-state transducers require a transformer to functionproperly. Disadvantageously, however, transformers are large, heavy, andexpensive, often costing more than the remaining transducer componentscombined. Thus the cost and size are often incompatible with desireduses.

Therefore a need exists for an improved replacement LVDT-type pressuretransducer that obviates the need for undesirable electro-mechanicalcomponents or transformers. It is to this need that the presentdisclosure addresses.

BRIEF SUMMARY OF DISCLOSURE

Exemplary embodiments of the present disclosure may provide an LVDT-typepressure transducer assembly, comprising a first and second inputterminal for receiving a first and second input voltage, respectively,wherein the first and second input voltages are about 180° relative toeach other, a Wheatstone bridge configured to receive the first andsecond input voltages, wherein the Wheatstone bridge is adapted tomeasure an applied physical parameter and output first and secondsignals substantially indicative of the physical parameter, and a firstoutput terminal and a second output terminal for outputting a first andsecond output voltage, respectively, that correspond to the first andsecond signal, respectively, wherein a sum of the first and secondoutput voltages remains relatively constant, and wherein the pressuretransducer assembly does not comprise a transformer component.

Other exemplary embodiments of the present disclosure may provide atransformer-less LVDT-type pressure transducer assembly, comprising afirst and second input terminal for receiving a first and second inputvoltage, respectively, a rectifying circuit assembly adapted to receivethe first and second input voltages, maintain the direction of the firstand second input voltages, and double the first and second inputvoltages, a sensing element configured to receive the first and secondinput voltages from the rectifying circuit, wherein the sensing elementis adapted to measure an applied pressure and output first and secondsignals substantially indicative of the pressure, and a first outputterminal and a second output terminal for outputting a first and secondoutput voltage, respectively, that correspond to the first and secondsignal, respectively, wherein the first and second output voltageschange relative to each other depending upon the direction and amount ofthe applied pressure.

Other exemplary embodiments of the present disclosure may provide amethod that includes receiving, at first and second input terminals,first and second input voltages, respectively, wherein the first andsecond input voltages are about 180° relative to each other and,responsive to receiving, at a rectifying circuit assembly in electricalcommunication with the first and second input terminals, the first andsecond input voltages, maintaining the direction of the first and secondinput voltages and doubling the first and second input voltages. Themethod may further include measuring, by a Wheatstone bridge, an appliedphysical parameter and outputting, by the Wheatstone bridge and to acapacitor, and to first and second operational amplifiers, first andsecond signals substantially indicative of the physical parameter.Moreover, the method may include reducing, by the capacitor, a DCcomponent of the first and second signals and amplifying, by the firstand second operational amplifiers, the first and second signals.Finally, the method may include adjusting, by a gain resistor inelectrical communication with the first and second operationalamplifiers, a differential output between first and second outputvoltages that correspond to the first and second signals output from theWheatstone bridge, respectively, and outputting, by a first outputterminal and a second output terminal, the first and second outputvoltages, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a linear displacement LVDT of the prior art.

FIG. 2 illustrates an LVDT-type pressure transducer assembly of theprior art.

FIG. 3 illustrates a solid-state LVDT-type transducer assembly of theprior art.

FIG. 4 illustrates the circuitry of an exemplary embodiment of theLVDT-type transducer assembly of the present disclosure.

FIGS. 5A and 5B graphically illustrate values of outputs V1 and V2versus pressure and their corresponding ratio, respectively, of theembodiment illustrated in FIG. 4.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Although preferred embodiments of the disclosure are explained indetail, it is to be understood that other embodiments are contemplated.Accordingly, it is not intended that the disclosure is limited in itsscope to the details of construction and arrangement of components setforth in the following description or illustrated in the drawings. Thedisclosure is capable of other embodiments and of being practiced orcarried out in various ways. Also, in describing the preferredembodiments, specific terminology will be resorted to for the sake ofclarity.

It must also be noted that, as used in the specification and theappended claims, the singular forms “a,” “an,” and “the” include pluralreferents unless the context clearly dictates otherwise.

Also, in describing the preferred embodiments, terminology will beresorted to for the sake of clarity. It is intended that each termcontemplates its broadest meaning as understood by those skilled in theart and includes all technical equivalents which operate in a similarmanner to accomplish a similar purpose.

By “comprising” or “containing” or “including” is meant that at leastthe named compound, element, particle, or method step is present in thecomposition or article or method, but does not exclude the presence ofother compounds, materials, particles, method steps, even if the othersuch compounds, material, particles, method steps have the same functionas what is named.

It is also to be understood that the mention of one or more method stepsdoes not preclude the presence of additional method steps or interveningmethod steps between those steps expressly identified. Similarly, it isalso to be understood that the mention of one or more components in adevice or system does not preclude the presence of additional componentsor intervening components between those components expressly identified.

Referring now to the drawings, in which like numerals represent likeelements, exemplary embodiments of the present disclosure are hereindescribed. It is to be understood that the figures and descriptions ofthe present disclosure have been simplified to illustrate elements thatare relevant for a clear understanding of the present disclosure, whileeliminating, for purposes of clarity, many other elements found intypical pressure sensor assemblies and methods of making and using thesame. Those of ordinary skill in the art will recognize that otherelements are desirable and/or required in order to implement the presentdisclosure. However, because such elements are well known in the art,and because they do not facilitate a better understanding of the presentdisclosure, a discussion of such elements is not provided herein.

Exemplary embodiments of the present disclosure provide atransformer-less, solid-state pressure transducer assembly havingsignificantly better characteristics than those of the prior art. Anexemplary embodiment of the pressure transducer assembly of the presentdisclosure is illustrated in FIG. 4. The pressure transducer assembly ofthe present disclosure achieves many of the same functions as transducerassemblies of the prior art having transformers, however the pressuretransducer assembly of the present disclosure is smaller in size andweight, costs less to manufacture, and has increased reliability.

To generate an accurate LVDT-type output, LVDT-type pressure transducerassemblies should satisfy certain requirements. First, the DC voltage(s)should be substantially minimized, if not removed as circuits thatmeasure LVDT type signals are sensitive to AC signals only, and aspurious DC component will interfere with their intended operation.Second, the pressure transducer circuitry should accommodate relativelylarge output voltages as the output voltages can reach about 5.5 VRMS,as illustrated in FIG. 5A. Third, the output voltages should have arelatively large bias (initial value at zero pressure) such that whenthe pressure increases, the decreasing voltage does not reach zero, asillustrated in FIG. 5A. Fourth, the sum of the two output voltagesshould remain relatively constant. Fifth, the phase shift of the twooutput voltages relative to the input voltage(s) should be substantiallyzero. The pressure transducer assembly of the present disclosuresatisfies these requirements without the use of a transformer, thereforemaking it a desirable replacement over prior art embodiments.

As illustrated in FIG. 4, the voltage supply of the pressure transducerassembly 100 is provided by two AC voltages VIN+ (105) and VIN− (110).In an exemplary embodiment, the input voltages VIN+ (105) and VIN− (110)can have a magnitude of about 3.5 VRMS. VIN− (110) may be about 180°relative to VIN+ (105), thus VIN−=−VIN+, therefore making VIN+ (105)about 3.5 VRMS and VIN− (110) about −3.5 VRMS. It should be understoodthat in other exemplary embodiments, the magnitude of the input voltagescan be higher or lower based on system requirements. For example and notlimitation, it may be desirable to have input voltages around about ±5.0VRMS. It shall be further understood that all voltages within thepressure transducer assembly 100, including the two output voltages V1(170) and V2 (175), which are labeled in this schematic as Vout+ andVout−, respectively, and will be further discussed herein, are measuredrelative to the pin RTN (180). Further, the phase reference of all thevoltages within the circuit is the phase of voltage VIN+ (105). Thus,the phase shift of the voltages within the pressure transducer assemblyis preferably substantially zero.

In prior art embodiments, such as the embodiment illustrated in FIG. 3,transformers were utilized as both a rectifying and doubling circuit,whereby the incoming voltage was doubled to enable better detection ofoutput voltages. The present disclosure, however, eliminates the needfor a transformer and replaces the transformer with a rectifying circuitassembly 115, as illustrated in FIG. 4. The rectifying circuit assembly115 of the present disclosure receives input voltages VIN+ (105) andVIN− (110) and comprises a plurality of diodes 120 and capacitors 125 inelectrical communication with each other and adapted to work together todouble the rectified voltages, as previously carried out by thetransformer in prior art embodiments, and maintain the positive andnegative voltages of the VIN+ (105) and VIN− (110), respectively. Inalternative embodiments, the voltage may be retained at near constantlevels or increased by a multiple other than two.

The rectifying circuit assembly 115 then feeds VIN+ (105) and VIN− (110)voltages to a sensing element, such as a Wheatstone bridge 130. TheWheatstone bridge 130 is adapted to measure an applied pressure andoutput voltage signals substantially indicative of the applied pressure,which are subsequently outputted as first and second output voltages V1and V2, which will be further discussed herein. The Wheatstone bridge130 comprises four piezoresistor elements arranged in two bridge legsthat are connected in parallel. Each bridge leg comprises twopiezoresistor elements connected in series, wherein one piezoresistorelement in each leg senses a physical parameter, such as pressure,applied in one direction, and the other piezoresistor element in eachleg senses the physical parameter, such as pressure, applied in theopposite direction. Thus, when pressure is applied in a particulardirection, the resistances of the corresponding pair of piezoresistorelements in each leg increase and correspondingly change the bridgeoutputs accordingly. The outputs from the Wheatstone bridge 130 areutilized to generate two voltage outputs, V1 (170) and V2 (175) from thepressure transducer assembly 100, wherein the applied pressure may bedetermined by calculating ratio R, which is defined as:

$R = \frac{{V\; 1} - {V\; 2}}{{V\; 1} + {V\; 2}}$Ratio R is proportional to the applied pressure. Thus, according to theequation, when no pressure is applied to the piezoresistor elements, theoutput V1 and V2 are equal and at a null point. Further, when there isan increase in pressure in one direction, output V1 increases whileoutput V2 decreases. Similarly, when there is an increase in pressure inthe opposite direction, output V1 decreases and output V2correspondingly increases. Thus, the output voltages V1 and V2 changerelative to each other depending upon the direction and amount of theapplied pressure, in a manner similar to the output voltages provided byan LVDT assembly. FIGS. 5A and 5B graphically illustrate the values ofthe two output voltages V1 (170) and V2 (175) and the ratio R relativeto pressure, respectfully, wherein “measurand” is the distance for LVDTdisplacement or input pressure LVDT-type pressure transducers.

As previously described, the Wheatstone bridge 130 may be supplied withAC input voltages from the rectifying circuit assembly, which have afrequency of about 3 kHz, such that the Wheatstone bridge 130 generatesa low-level AC voltage that is both proportional with the pressure andratiometric with the input voltages. It shall be understood that the twooutput voltages V1 (170) and V2 (175) are also AC voltages, are of thesame frequency, and are substantially in-phase with the input voltages.

The Wheatstone bridge 130 outputs may be amplified by aninstrumentation-type differential amplifier implemented via first andsecond operational amplifiers, U1A (135) and U1B (140), respectively,before they are outputted as first and second output voltages V1 (170)and V2 (175), respectively. Prior art embodiments, such as theembodiment illustrated in FIG. 3, utilize four operational amplifiers.The pressure transducer assembly 100 of the present disclosuresimplifies this circuitry as it utilizes two operational amplifiers togenerate a sufficient voltage level. Further, the first and secondoperational amplifiers 135/140 are in electrical communication with gainresistor RGAIN (155), which enables the first and second operationalamplifiers 135/140 to output what four operational amplifiers of theprior art outputted, and adjusts the V1 (170) and V2 (175) outputs sothat they maintain a relatively constant V1+V2 sum. It shall beunderstood that RGAIN (155) may be a variable resistor component or aset resistor component. Further, the ratio of resistors R7 (145) and R5(150), which are in parallel with RGAIN (155), enable RGAIN (155) todetermine the gain adjustment needed to deliver a sufficient voltagesignal to the V1 (170) and V2 (175) outputs and maintain the V1+V2 sum.The gain resistor RGAIN (155) operates as a function of the bridgeoutput, thus the differential output (V1−V2) may be adjusted so theratio R is of a desired value. This is possible because this sum of thetwo output voltages V1+V2 remains relatively constant, independent ofthe pressure, and irrespective of the value of the gain resistor RGAIN.Embodiments of the prior art do not utilize such an RGAIN (155)component. Rather, prior art embodiments utilize separate resistors thatadjust at the same time, and one skilled in the art will appreciate thedifficulty in adjusting two parameters simultaneously.

Further, an additional bias voltage common for both output voltages V1and V2 may be generated by resistors R3 (160) and R4 (165). In such anembodiment, the value of the bias voltage is equal to −(VIN−)(R7/R3). AsVIN− is equal to −VIN+, the bias voltage equals (VIN+)(R7/R3). Thevalues of the resistors may be chosen such that the two output voltagesV1 (170) and V2 (175) have a relatively large bias and maintains arelatively constant V1+V2 sum.

To maintain a negligible DC component for the output voltages V1 and V2(as previously described, the output voltages are AC voltages), the gaincircuit uses capacitor C7 (185). Capacitor C7 (185) acts like a shortcircuit at the operating frequency of about 3 kHz, but has a very large,practically infinite, resistance at DC. As a result, the AC gain isunaffected by capacitor C7 (185) and can be reduced to unity (x1) at DC.Theoretically, this feature is not necessary as the DC output of thebridge is zero, and the operational amplifiers have zero offset. In manyenvironments, however, these voltages are not completely zero, andwithout capacitor C7 (185), the DC component would be amplified by thesame gain as the AC path. Thus, capacitor C7 (185) substantially reducesthe DC component such that it becomes a negligible factor, and issubstantially equal to the offset voltage of the operational amplifiers.

Additionally, the first and second operational amplifiers U1A (135) andU1B (140) are supplied with two DC voltages, which may be generated bythe rectifiers/voltage doublers implemented with the diodes D1A, D1B,D2A, D2B, D3A, D3B, D4A, D4B, and capacitors C1, C2, C3 and C4.Capacitors C5 and C6 may filter out ripple, resulting in stable DCsupply voltages for U1A (135) and U1B (140). This implementation allowsfor the generation of relatively high value output voltages V1 (170) andV2 (175), without the use of transformers.

It will be apparent to those skilled in the art that modifications andvariations may be made in the apparatus and process of the presentdisclosure without departing from the spirit or scope of the disclosure.It is intended that the present disclosure cover the modification andvariations of this disclosure provided they come within the scope of theappended claims and their equivalents.

The invention claimed is:
 1. A method comprising: receiving, at firstand second input terminals, first and second input voltages,respectively, wherein the first and second input voltages are about 180°relative to each other; responsive to receiving, at a rectifying circuitassembly in electrical communication with the first and second inputterminals, the first and second input voltages, maintaining thedirection of the first and second input voltages and doubling the firstand second input voltages; measuring, by a Wheatstone bridge, an appliedphysical parameter; outputting, by the Wheatstone bridge and to acapacitor, and to first and second operational amplifiers, first andsecond signals substantially indicative of the physical parameter;reducing, by the capacitor, a DC component of the first and secondsignals; amplifying, by the first and second operational amplifiers, thefirst and second signals; adjusting, by a gain resistor in electricalcommunication with the first and second operational amplifiers, adifferential output between first and second output voltages thatcorrespond to the first and second signals output from the Wheatstonebridge, respectively; and outputting, by a first output terminal and asecond output terminal, the first and second output voltages,respectively.
 2. The method of claim 1, wherein the first and secondinput voltages are AC voltages.
 3. The method of claim 1, wherein thefirst and second input voltages are about +3.5 VRMS and −3.5 VRMS,respectively.
 4. The method of claim 1, wherein the first and secondinput voltages have a frequency of about 3 kHz.
 5. The method of claim4, wherein the first and second output voltages are substantially thesame frequency as the first and second input voltages, respectively. 6.The method of claim 1, wherein the first and second output voltages aresubstantially in-phase with the first and second input voltages,respectively.
 7. The method of claim 1, wherein the first and secondoutput voltages change relative to each other depending upon thedirection and amount of the applied physical parameter.
 8. The method ofclaim 1, wherein the physical parameter is pressure.
 9. The method ofclaim 1, wherein a sum of the first and second output voltages remainsrelatively constant.
 10. The method of claim 1, wherein the gainresistor operates as a function of the first and second signals outputfrom the Wheatstone bridge.
 11. A method comprising: receiving, at firstand second input terminals, first and second input voltages,respectively; responsive to receiving, at a rectifying circuit assemblyin electrical communication with the first and second input terminals,the first and second input voltages, maintaining the direction of thefirst and second input voltages and doubling the first and second inputvoltages; measuring, by a Wheatstone bridge, an applied pressure, theWheatstone bridge included in a sensing circuit; outputting, by theWheatstone bridge and to first and second differential amplifiers, firstand second signals substantially indicative of the applied pressure;receiving, by the first and second differential amplifiers,respectively, the first and second input voltages from the rectifyingcircuit assembly; adjusting, by a gain resistor in electricalcommunication with the first and second differential amplifiers, adifferential output between first and second output voltages thatcorrespond to the first and second signals, and wherein the gainresistor operates as a function of the first and second signalsoutputted from the Wheatstone bridge; and outputting, by a first outputterminal and a second output terminal, the first and second outputvoltages, respectively, wherein the first and second output voltageschange relative to each other depending upon the direction and amount ofthe applied pressure.
 12. The method of claim 11, wherein the first andsecond input voltages are AC voltages.
 13. The method of claim 11,wherein the first and second input voltages are about 180° relative toeach other.
 14. The method of claim 11 wherein the first and secondinput voltages are about +3.5 VRMS and −3.5 VRMS, respectively.
 15. Themethod of claim 11, wherein the first and second input voltages have afrequency of about 3 kHz.
 16. The method of claim 15, wherein the firstand second output voltages are substantially the same frequency as thefirst and second input voltages, respectively.
 17. The method of claim11, wherein the first and second output voltages are substantiallyin-phase with the first and second input voltages, respectively.